1. Field of the Invention
The present invention, relates to high resolution ultrasound imaging of small structures at high frequencies, typically above 5 MHz, where the ultrasound transducer or transducer array is brought close to the structure to be imaged through channels with limited space for cable wires connecting the ultrasound transducer(s) and the ultrasound imaging or measurement instrument.
Examples of such applications are intravascular ultrasound imaging (IVUS) of a vessel wall from a transducer at the tip of a catheter, intraurether imaging of the prostate, or high resolution imaging of tumors and small vessels during minimal invasive or other surgery through narrow channels. The invention presents solutions with particularly high signal to noise ratio of the measured back scattered signal, with high inertness to electromagnetic interference from external sources, in such situations.
More specific, the invention relates to a design of preamplifier electronics, circuits for acoustic beam forming with ultrasound transducer arrays, ultrasound transducer arrays, and combinations thereof, that allows the electronics and transducer(s) to be integrated with short distance in a compact assembly, that can be operated from an ultrasound imaging or measurement system with a small number of electric wires, down to a two-wire cable.
The invention also has applications for obtaining maximal signal to noise ratio and inertness to electromagnetic interference with high frequency ultrasound imaging of structures with simpler access, such as high resolution skin or eye imaging (fxcx9c20-100 MHz).
It further has applications with lower frequency imaging and measurements for transducer arrays with small elements with high electric impedance, to improve the signal to noise ratio and the inertness to electromagnetic interference in these cases. It also has applications for switched elevation focusing with linear arrays, to reduce the number of wires connecting to the instrument, for easier manual operation of the transducer array.
2. Description of the Related Art
The spatial resolution with ultrasound echo imaging systems is a couple of ultrasound wavelengths large. The ultrasound wavelength, xcex, is related to the ultrasound frequency, f, as xcex=c/f, where cxcx9c1540 xcexcm/xcexcs is the propagation velocity of ultrasound in the tissue. To obtain a low wavelength, and hence a sharp resolution, one must therefore use a high ultrasound frequency. However, the image depth with ultrasound echo imaging is limited by absorption of ultrasound energy in the tissue. As the absorption increases with frequency, this sets an upper limit on the frequency that can be used for a given image depth. The image resolution is therefore limited by the image depth.
For imaging of small structures like the vessel wall, or other small structures of internal organs, it is therefore necessary to bring the ultrasound transducer close to the structure, so that the image depth, and hence the absorption attenuation of the ultrasound, is limited. With image depths less than 10 mm, it is possible to use ultrasound frequencies in the 20-100 MHz range, with wave lengths ranging from 75 to 15 xcexcm. This gives spatial resolution in the range of around 150 xcexcm to 30 xcexcm, depending on the transducer frequency, bandwidth, and aperture.
The ultrasound transducer can be brought close to internal structures in the body, like the vessel wall or other organs, by mounting the transducer structure at the tip of a catheter or other elongated devices, that are inserted into the body through an incisure or natural body openings. A cable then connects the transducer at the tip of the extended probe and the ultrasound imaging or measurement instrument. The use of such high frequencies with transducers that has a distance to the imaging instrument then introduces practical problems, as:
1. With ultrasound frequencies above 30 MHz, impedance mismatch between the transducer and the cable connecting to the imaging instrument introduces losses that limits the signal to noise ratio and hence the maximal image depth at a given frequency. The limited thickness of the insertion instrument also limits the thickness of the wires that connects the transducer and the imaging instrument giving additional absorption and reduced imaging sensitivity.
2. At for example 35 MHz, the electromagnetic wavelength in the cable is xcx9c6 m, giving a quarter wavelength of xcx9c1.5 m that is approximately the length of a typical catheter. The catheter hence becomes similar to a quarter wave tuned antenna in the ultrasound receiver frequency range, and the imaging system becomes very sensitive to external electromagnetic interference (EMI) sources in the active receiver frequency range.
3. Other problems in using a thin cable between the ultrasound transducer and the instrument is related to obtaining narrow ultrasound beams. To reduce the effect of ultrasound wave diffraction in the beam focus, and hence reduce the focal diameter, one must have a large number of wavelengths across the active transducer aperture (typically xcx9c50 wavelengths across the aperture are wanted). However, with such low diffraction focusing, the depth of the focus is reduced, limiting the range of sharp focusing and spatial resolution.
The standard solution to this problem for the receive beam, is to use an array of transducer elements with dynamic focusing where the receive beam focus follows the depth where the echoes are received from at any time. An electronically steered dynamic focus is obtained by adding delays to each array element signal, so that the total of this delay and the propagation delay from the focus to the element, is close to the same for all elements. The added delay can be obtained with acoustic or electronic delay lines, or a combination of both. One also wants to increase the active transducer transmit aperture with image depth, in order to limit expansion of the focal diameter with depth.
4. For pulse transmission one must select a fixed transmit beam focus, as one cannot change the pulse after it is transmitted. It is then desirable to be able to select between different transmit focus depths so that one can focus the transmit beam to the most important image range. A kind of dynamic focus for the transmit beam can be obtained by composing the whole image range of sub ranges where each sub range is imaged with separate transmit pulses focused within the sub range. One also wants to increase the active transducer transmit aperture with image depth, in order to limit expansion of the focal diameter with depth.
Hence, it is desirable to have a transducer array at the distal end of the insertion device that operates with a high signal to noise ratio with large immunity to electromagnetic interference, the array having dynamic or switchable receive focusing and expanding receive aperture, switchable transmit focusing and expanding transmit aperture, that can be operated from the ultrasound imaging or measurement instrument via a minimal number of wires, minimizing the cross section of the device to be inserted into narrow structures.
The invention devices a solution to these problems by mounting electronic circuits close to the ultrasound transducer or transducer array, where the circuits have the ability to be operated through a few wires, down to a two-wire cable.
In its simplest form, the invention provides a preamplifier that can be operated through a two wire cable that provides the DC bias voltage to the amplifier. When a high voltage pulse is transmitted via the wire, a breakthrough circuit connects the wire to the transducer for transmit of the ultrasound pulse, while in receive mode, the low level signal on the transducer is amplified and fed as a higher level signal via the same wire to the imaging or measurement instrument. As the receive signal level is raised on the cable, the system is less sensitive to cable losses and external electromagnetic interference, hence maximizing the sensitivity of the imaging or the measurement.
For dynamic receive focusing with an array, the invention provides in its most general form an integrated electronic circuit to be mounted close to the transducer array, the circuit providing preamplifiers for the individual elements and delay circuits that are automatically switched in a time sequence after the pulse transmission, so that a dynamic focusing of the receive beam is obtained. In one embodiment of the invention, an acoustic delay line in front of the transducer element is used for the full or partial delay of the element signal. The circuit can also be set up so that the transmit pulses select different transmit foci in a sequence so that multiple transmit focus imaging can be obtained. The invention also opens for the use of coded signaling prior to the transmit pulse to select the transmit focus setting as well as the dynamic receive focusing range via a single wire.
In a simpler implementation, the invention achieves steered focus and aperture with a two wire cable between the imaging instrument and the transducer system by using a pre-programmed amplifier and switching circuit. The switching circuit selectively combines a set of pre-focused array elements with preset delays, for each focal range. The preset focus and delay for each element is selected so that the phase error both across each element and across the active aperture, is less than a limit, say 90-120 deg, for the range the element participates to the beam forming. Combining an increasing number of elements with depth, one obtains an increasing aperture that limits the beam diameter with the increasing depth.
In a particularly simple implementation, both the pre focusing and delay of each element are provided with acoustic lens material with adapted curvature and thickness in front of each element. The electronic circuit then provides possible amplification of the signal from each element before the signals are selectively added in a time steered switching circuit that is reset by the pulse transmission.